Mars Express and the
Story of Water on Mars
(16 October 2006) For a number of
decades now, astronomers have wondered about water on Mars. Thanks to ESA's
Mars Express, much of the speculation has been replaced with
facts.
Launched on 2 June 2003, Mars Express has changed the way
we think of Mars.
Since the Viking missions of the 1970s, planetary
scientists have changed their perception of water on Mars several times,
passing from the picture of a dry planet to that of a warmer and wetter one.
Mars Express's data are now shedding a new light on the complex issue of the
evolution of water on the Red Planet.
This image taken by the High Resolution Stereo Camera (HRSC) on board ESA's Mars Express, shows a perspective view of a glacial feature located in Deuteronilus Mensae. The image is centred at a 37.92º North latitude and 24.61º East longitude (courtesy: ESA/DLR/FU Berlin (G. Neukum))
"We are re-writing the history of Mars,"
says Gerhard Neukum, Freie Universitaet Berlin, Germany, and the Principal
Investigator on Mars Express's High Resolution Stereo Camera (HRSC). "The big
picture of a warm wet Mars is not completely correct. Any warm wet period
lasted only a few hundred million years. By four thousand million years ago, it
was over," he adds.
Three instruments on Mars Express have been at the
centre of this revolution in thought. One is the Mars Advanced Radar for
Subsurface and Ionospheric Sounding (MARSIS). Since July 2005, MARSIS has
probed beneath the surface of Mars to depths of thousands of metres. This is
the first time such investigations have taken place.
"MARSIS has shown
that many of the upper layers of Mars contain water ice," says Jeffrey Plaut of
the Jet Propulsion Laboratory, Pasadena, who is the co-Principal Investigator
on the MARSIS experiment.
The scientists detected abundant water ice in
the Martian polar regions and also received a surprise from some of the very
first results that MARSIS returned. When the radar passed over the mid northern
latitudes of Chryse Planitia, the signals showed a buried impact crater, below
the surface. Inside this impact structure was a thick layer of possibly
water-ice-rich material. "We are finding reservoirs of ice that have never been
seen before," says Plaut, "But we are still puzzling out when and where the
water on Mars was liquid."
"The last MARSIS observations have been done
on the South Pole," adds Giovanni Picardi, MARSIS Principal Investigator, from
the University of Rome 'La Sapienza'. "The quality of the preliminary results
of the advanced analysis we are still performing are really exciting and
promising, with respect to the main scientific objectives of our experiment."
The objectives include the detection of subsurface water.
In this HRSC 3D perspective view of the Marwth Vallis area (shades of grey), OMEGA has mapped the water-rich minerals (blue). No hydrated minerals or sediments have been detected, either in the channel or in its opening. However, the outflow was so violent as to erode and expose ancient hydrated clay-rich minerals, tracing an early era when water was present. (courtesy: ESA/OMEGA/HRSC)
The OMEGA Visible and Infrared Mineralogical
Mapping Spectrometer has taken giant steps towards answering that question.
OMEGA detects minerals on the surface of Mars. Three in particular reveal the
history of Martian water. "We have demonstrated that water could have been
stable on Mars's surface but not for very long," says Jean-Pierre Bibring,
Institut d'Astrophysique Spatiale, Orsay, France, and OMEGA's Principal
Investigator.
OMEGA detected clay-like minerals that form during
long-term exposure to water, but only in the oldest regions of Mars. That
suggested water flowed during the first few hundred million years of the
planet's history only. When these bodies of water were lost, water then
occasionally burst from inside the planet but quickly evaporated.
During
the evaporation they made sulphates, the second mineral that OMEGA detected.
When even this stopped and the remaining water on Mars became permanently
frozen, then the atmosphere gradually turned the soil red by creating the third
mineral OMEGA detected, ferric oxide.
Mars has been like this for
thousands of millions of years. "It is remarkable that, for the first time, we
have identified where and when liquid water might have been present on Mars. It
is not where one thought of before," says Bibring.
This HRSC image shows the Northern main channel of Kasei Valles, which probably has been formed by gigantic flood events. The image is centred at 26.97º North latitude and 67.68º West longitude. (courtesy: ESA/DLR/FU Berlin (G. Neukum))
The images from the High Resolution Stereo Camera (HRSC) point towards the same conclusions. They show the Martian surface in the most exquisite detail, revealing features just 10 metres across. They clearly show extremely old Martian regions that have been eroded by flowing water. The pictures also show a huge valley, Kasei Valles, carved by a gigantic Martian glacier that persisted for a thousand million years during the time when the temperature of Mars had dropped too low for liquid water to flow across the surface.
This HRSC image provides a perspective view of residual water ice on the floor of Vastitas Borealis Crater on Mars. The image is centred at 70.17º North latitude and 103.21º East longitude. (courtesy: ESA/DLR/FU Berlin (G. Neukum))
"We see a clear link between volcanic
regions and water flows," says Neukum. Wherever there has been volcanic
activity on Mars, it has melted water inside Mars and let it flow to the
surface. Some of these flows are recent - geologically speaking. "At the foot
of Olympus Mons, HRSC sees evidence for water flows that have happened within
the last 30 million years," says Neukum.
NASA's latest spacecraft, the
Mars Reconnaissance Orbiter (MRO), carries instruments that lead on from those
of Mars Express. Many scientists from the teams at work on MARSIS are now
working on the ASI's Shallow Radar (SHARAD) on board MRO. This is tuned to
focus on the shallower layers of Mars, whereas MARSIS looks deeper. OMEGA's
sister instrument on MRO is the Compact Reconnaissance Imaging Spectrometer for
Mars (CRISM). This will look in more detail at minerals on the Martian surface.
However, the instrument only has a small field of view, so it will need
guidance. "They will target primarily the areas that OMEGA has shown to be
interesting," says Bibring.
"Mars Express has provided unprecedented
evidence on the history of water on Mars. Now, we look forward to new
investigations that will build on this legacy," says Augustin Chicarro, Mars
Express's Project Scientist at ESA.
This HRSC image shows a perspective view of a possible dust-covered frozen sea near the Martian equator. The image is centred at 5.46º North latitude and 150.30º East longitude. (courtesy: ESA/DLR/FU Berlin (G. Neukum))
Mars Express data is still streaming down
from HRSC, MARSIS, Omega but also the probe's other instruments, PFS, SPICAM,
ASPERA, and MaRS. They are probing all aspects of the Martian environment -
studying atmospheric gases, searching for eventual biological processes,
detecting high altitude clouds and hidden volcanoes and digging into the
scavenging effects of the solar wind.
(source: ESA)